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Selecting the Right Filter for Wireless Handset Applications

Fri, 03/06/2009 - 6:55am
As more and more bands are getting integrated into wireless devices, rejecting the transmitter signals of the other radio bands becomes crucial when using a common antenna.

By Allen Chien, Ph.D., Wireless Semiconductor Division, Avago Technologies


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Figure 1. Schematic showing the center of the edge channels for GSM, CDMA, and UMTS filters. This is where the IL specification should be identified for each protocol.
In every wireless handset you will find numerous filters and/or duplexers present in the RF front end. Filters fall into four general categories: band-pass, band-reject, high pass and low pass. The most common type used in handset applications are band-pass filters. The function they perform is to pass signals only in the desired frequency range and reject others outside that range. Many different technologies can be used to make band-pass filters, including lumped element LC, dielectric, cavity, SAW and FBAR filters. The technology you end up selecting will depend upon the end application. On the infrastructure and consumer appliance side, devices are usually "larger" so cavity or dielectric filters are generally selected because they provide the best performance for the size. On the mobile device side, devices are generally "smaller and handheld." SAW and FBAR filters are the two technologies of choice here. LC filters are implemented directly in the PCB, but have poor performance compared to the other technologies and are used when filtering requirements are not as stringent. This article will address several topics that customers looking to purchase filters for wireless handset applications should consider.
Insertion Loss
Insertion loss is often called "the money spec" because it has a direct impact on talktime. Insertion loss (IL) for filters is defined as the signal loss of including the filter in the system and expressed as a ratio in dB relative to the transmitted signal power before insertion of the device. Exactly how much IL filters can achieve depends upon the technology and application. Low IL values are hard to achieve when the bandwidth of the filter is wide and the transmit/receive bands are close together. For example, the CDMA PCS band has a high percent bandwidth and close transmit (Tx)/receive (Rx) spacing. Typical max. IL values one can expect range from 2.5 to 3.5 dB depending upon technology. Here, FBAR devices are used widely due to it's higher intrinsic quality factors providing superior performance. Compare this to Cell band filters where 2 dB max. values are typical due to easier percent bandwidth and spacing requirements. Here, SAW filters are widely used due to low price and availability from multiple suppliers. Pay attention to the frequency and temperature range quoted on the datasheet during product selection to make sure it matches your application. Filters for UMTS handsets generally specify a frequency range of 5 MHz less than CDMA and GSM filters as shown in Figure 1. In addition, the temperature ranges for specified for UTMS, GSM and CDMA filters all differ, so make sure that the worst case temperature range is specified if your device is using multiple standards.
Isolation

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Figure 2. PCS/Cell/GPS multiplexer with five FBAR filters attached to a single antenna output.
The amount of attenuation between the Tx and Rx ports of a duplexer is known as isolation. This differs from rejection, which is the amount of attenuation between the Tx or Rx port and the antenna port. Rx band isolation of the duplexer is important to block the thermal noise coming out from the power amplifier (PA) and the baseband chip. Tx band isolation is important because it blocks the large PA Tx signal from leaking into the Rx chain. Those leakages can cause reciprocal mixing, cross modulation, and IMD2 products at the mixer or LNA which generates noise and ends up degrading system sensitivity. In CDMA systems, this is especially important to maintain good single tone desense (STD) performance. Pay attention to the over-temperature isolation performance when considering duplexer performance, as it can effect the corner channel sensitivity quite significantly. Forward looking, there is a trend to remove Tx and Rx interstage filters from the RF front end. In order to do this, duplexers with high common mode isolation values, such as 55 dB in the Tx and 50 dB in the Rx band, are being asked for by baseband chip suppliers.
Out of Band Rejection
Out-of-band rejection of a filter is important for all protocols to minimize spurious/out-of-band emissions radiated from the phone as well as jammers from within. For WCDMA devices, 3GPP regulations specify that the system can not emit more than -30 dBm of out-of-band power at the antenna. So here, second and third harmonic rejection of the Tx chain are the important specifications to pay attention to. As more and more bands are getting integrated into wireless devices, rejecting the transmitter signals of the other radio bands becomes crucial when using a common antenna. In a mobile phone with quad-band GSM, tri-band UMTS, GPS, and WiFi, there are nine different possible jammers. Sufficiently rejecting the Tx signal from any transmitter in operation is important so that the Rx chains of the other frequencies can still listen to the base station signals. In the case of GPS and WiFi, concurrent operation is possible when a phone call is in progress. For example, tri-band dual-mode CDMA phones using a single antenna is common in the U.S. today. GPS will work concurrently with the transmit radios (PCS & cell band), and PCS/Cell-band Tx signal leakage and Tx + Jammer cross-modulations signals become important new out-of-band jammers to attenuate. Rejection of these signals is critical to prevent LNA gain compression and preserve good GPS system sensitivity.
Size and Differential Output
Figure 3. Handset duplexer size trends for the last five years.
Figure 3 shows the trend in duplexer size reduction for the past five years. This trend of size reduction will continue with most filter suppliers coming out with new products using a common footprint of 2 × 2.5 mm in 2009 and 2 × 1.6 mm for 2010 and beyond. Interstage Tx and Rx filters are also shrinking to a standard footprint of 1.1 × 0.9 mm for single filters and 1.5 × 1.1 for dual filters. These filters could slowly die out though with the introduction of high-isolation duplexers. Elimination of interstage filters will reduce the overall BOM cost and enable the use of multiband power amplifiers on the horizon. There is also a trend towards differential output of the Rx side of the duplexer. The reason for this is that many transceiver companies have moved to differential inputs in the receive chain to achieve better system sensitivity.
Integration
The vast majority of all duplexers used in wireless handsets are still discrete. Integration of the duplexers in a multiband radio makes lots of sense to reduce space, improve performance and simplify design for faster time to market. However, integrating also means lower yields for component suppliers and reduced flexibility in device layout. As examples of effective integration, PCS/Cell/GPS multiplexers are very common in U.S. CDMA phones. Here, a standard radio BOM enables consolidation of these three duplexers/filters into a standard single package. Five FBAR filters are attached to a single antenna output, Figure 2, allowing the end user to improve performance, routing, and matching into a single integrated device. Combining GSM Rx filters together into 2-in-1, 3-in-1 and 4-in-1 filter modules are common place in GSM phones. We are now seeing these GSM Rx filter banks being integrated with switches to form antenna switch modules. On the horizon, UMTS duplexers look to be the next piece being integrated into these antenna switch modules as 3G phones become more and more common place.

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